Cesar R. S. da Silva

The composition and geotherm of the lower mantle: constraints from the elasticity of silicate perovskite

Abstract A newly developed parameterization of the third-order isentropic finite strain equation of states (EOS) is used in conjunction with experimental data and theoretical results on MgSiO3 perovskite. New geotherms for the lower mantle are derived by comparison with preliminary reference earth model (PREM). The geotherms are adiabatic up to 1500 km depth and super-adiabatic thereafter. A description of the critical steps in obtaining the new parameterization is also given.

Elastic constants and anisotropy of forsterite at high pressure

We have determined from first principles the athermal elastic constant tensor of Mg2SiO4 forsterite with the plane wave pseudopotential method over a wide range of pressure (0–100 GPa) that encompasses the full range over which forsterite has been observed experimentally. The computed elastic constants are in excellent agreement with experimental data up to the maximum pressure of the experiments (16 GPa). We calculate the single-crystal elastic anisotropy from the elastic constants. We find that the anisotropy is strong (azimuthal P- and S-wave anisotropy: 25% and 20%, respectively, polarization anisotropy: 15%), in agreement with experiment, and that it depends weakly on pressure over the range 0–25 GPa, in contrast to the behavior of other silicates and oxides.

Vibrational and thermodynamic properties of forsterite at mantle conditions

[1] We present a first-principle study of the vibrational and thermodynamic properties of Mg2SiO4 forsterite up to 20 GPa. The calculated local density approximation (LDA) frequencies and their pressure dependence are in good agreement with the available Raman and infrared spectroscopy data. We also predict the pressure dependence of the modes which are yet to be measured. Thermodynamic properties are obtained using the quasi-harmonic approximation (QHA) to the free energy in conjunction with these results. The calculated pressure-volume-temperature (P-V-T) relations and thermodynamic properties agree well with the reported experimental data within the regime of validity of the QHA. The only discrepancies with experimental data that point to intrinsic anharmonic effects are the mode Gruneisen parameters of two Raman and one infrared modes. However, their effect on thermodynamic properties appears to be negligible. Citation: Li, L., R. M. Wentzcovitch, D. J. Weidner, and C. R. S. Da Silva (2007), Vibrational and thermodynamic properties of forsterite at mantle conditions, J. Geophys. Res., 112, B05206, doi:10.1029/2006JB004546.

Ab initio study of the elastic behavior of MgSiO3 ilmenite at high pressure

We investigate the athermal high pressure be- havior of the elastic properties of MgSiO3 ilmenite up to 30 GPa using the ab initio pseudopotential method. Our results at zero pressure are in good agreement with single- crystal elasticity measurements. The elastic anisotropy is shown to decrease slightly under compression and hence to remain substantial (25 to 20% shear wave anisotropy and 16 to 10% longitudinal wave anisotropy) over the pressure regime studied. The directions of fastest and slowest wave propagation are found to change slightly with pressure as determined by the pressure dependence of c14 and c25 .C om- parisons with the elastic behavior of other deep transition zone phases such as ringwoodite and garnet show that il- menite is likely to be the fastest and most anisotropic min- eral in this region. Large contrasts ( 10%) in velocities and densities between ilmenite and garnet are suggested to be signicant for the interpretation of lateral structure in the transition zone.

Metadata management for distributed first principles calculations in VLab-A collaborative cyberinfrastructure for materials computation

Abstract This paper describes the metadata and metadata management algorithms necessary to handle the concurrent execution of multiple tasks from a single workflow, in a collaborative service oriented architecture environment. Metadata requirements are imposed by the distributed workflow that calculates thermoelastic properties of materials at high pressures and temperatures. The scientific relevance of this workflow is also discussed. We explain the basic metaphor, the receipt, underlying the metadata management. We show the actual java representation of the receipt, and explain how it is converted to XML in order to be transferred between servers, and stored in a database. We also discuss how the collaborative aspect of user activity on running workflows could potentially lead to race conditions, how this affects requirements on metadata, and how these race conditions are precluded. Finally we describe an additional metadata structure, complementary to the receipts, that contains general information about the workflow.

VLab: collaborative Grid services and portals to support computational material science

We present the initial architecture and implementation of VLab, a Grid and Web‐Service‐based system for enabling distributed and collaborative computational chemistry and material science applications for the study of planetary materials. The requirements of VLab include job preparation and submission, job monitoring, data storage and analysis, and distributed collaboration. These components are divided into client entry (input file creation, visualization of data, task requests) and back‐end services (storage, analysis, computation). Clients and services communicate through NaradaBrokering, a publish/subscribe Grid middleware system that identifies specific hardware information with topics rather than IP addresses. We describe three aspects of VLab in this paper: (1) managing user interfaces and input data with JavaBeans and Java Server Faces; (2) integrating Java Server Faces with the Java CoG Kit; and (3) designing a middleware framework that supports collaboration. To prototype our collaboration and visualization infrastructure, we have developed a service that transforms a scalar data set into its wavelet representation. General adaptors are placed between the endpoints and NaradaBrokering, which serve to isolate the clients/services from the middleware. This permits client and service development independently of potential changes to the middleware. Copyright

First principles investigation of the A7 to simple cubic transformation in As

Abstract We investigate by means of first principles variable cell shape molecular dynamics (FP-VCS-MD) the pressure induced A7 to simple cubic (sc) phase transformation in As. The major experimental facts associated with this phase change and with the individual phases are very well reproduced, including the weakly first order character of the transformation. We also report a visual animation of the atomic motion and charge density evolution during the structural change caused by: (a) a sudden compression of the A7 phase to 30 GPa, and (b) a stepwise compression up to 30 GPa at T = 300 K. The goal of this animation was to shed light into the nature of the trajectories generated by the fictitious VCS-MD.

Correction to ''Vibrational and thermodynamic properties of forsterite at mantle conditions''

[1] In the paper ‘‘Vibrational and thermodynamic properties of forsterite at mantle conditions’’ by Li Li, Renata M. Wentzcovitch, Donald J. Weidner, and Cesar R. S. Da Silva (Journal of Geophysical Research, 112, B05206, doi:10.1029/2006JB004546, 2007), Figure 3 was misdrafted. As a result, the region below 760 K is marked invalid for quasi-harmonic approximation (QHA). But this error does not alter the results nor the interpretation of the paper for the Earth since the geotherm is within the QHA valid region. Figure 3 is redrawn.